The term "reversal photographic element" designates a photographic element which produces a photographic image for viewing by being imagewise exposed and developed to produce a negative of the image to be viewed, followed by uniform exposure and/or fogging of residual silver halide and processing to produce a second, viewable image. Color slides, such as those produced from Kodachrom.TM. and Ektachrome.TM. films, constitute a popular example of reversal photographic elements. In the overwhelming majority of applications the first image is negative and the second image is positive.
Although tabular grains had been observed in silver bromide and bromoiodide photographic emulsions dating from the earliest observations of magnified grains and grain replicas, it was not until the early 1980's that photographic advantages, such as improved speed-granularity relationships, more rapid developability, increased thermal stability, increased separation of blue and minus blue imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, were realized to be attainable from silver halide emulsions in which the majority of the total grain population based on grain projected area is accounted for by tabular grains satisfying the mean tabularity (T) relationship: EQU D/t.sup.2 &gt;25
where
D is the equivalent circular diameter (ECD) in micrometers of the tabular grains and PA1 t is the thickness in micrometers of the tabular grains.
Once photographic advantages were demonstrated with tabular grain silver bromide and bromoiodide emulsions techniques were devised to prepare tabular grains containing silver chloride alone or in combination with other silver halides.
Notwithstanding the many established advantages of tabular grain emulsions, the art has observed that these emulsions tend toward more disperse grain populations than can be achieved in the preparation of regular, untwinned grain populations--e.g., cubes, octahedra and cubo-octahedral grains. This has been a concern in some, but not all, photographic applications for tabular grain emulsions.
In the earliest tabular grain emulsions dispersity concerns were largely focused on the presence of significant populations of nonconforming grain shapes among the tabular grains conforming to the aim grain structure. While the presence of nonconforming grain shapes in tabular grain emulsions has continued to detract from achieving narrow grain dispersities, as procedures for preparing tabular grains have been improved to reduce the inadvertent inclusion of nonconforming grain shapes, interest has increased in reducing the dispersity of the tabular grains.
A technique for quantifying grain dispersity that has been applied to both nontabular and tabular grain emulsions is to obtain a statistically significant sampling of the individual grain projected areas, calculate the corresponding ECD of each grain, determine the standard deviation of the grain ECDs, divide the standard deviation of the grain population by the mean ECD of the grains sampled and multiply by 100 to obtain the coefficient of variation (COV) of the grain population as a percentage. While very highly monodisperse (COV&lt;10 percent) emulsions containing regular nontabular grains can be obtained, even the most carefully controlled precipitations of tabular grain emulsions have rarely achieved a COV of less than 20 percent. Research Disclosure, vol. 232, Aug. 1983, Item 23212 (Mignot French Patent 2,534,036, corresponding) discloses the preparation of silver bromide tabular grain emulsions with COVs ranging down to 15. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire P010 7DQ, England.
Saitou et al U.S. Pat. No. 4,797,354 reports in Example 9 a COV of 11.1 percent; however, this number is not comparable to that reported by Mignot. Saitou et al is reporting only the COV within a selected tabular grain population. Excluded from these COV calculations is the nonconforming grain population within the emulsion, which, of course, is the grain population that has the maximum impact on increasing grain dispersity and overall COV. When the total grain populations of the Saitou et al emulsions are sampled, significantly increased COVs (well in excess of 20%) result.
Techniques for quantitatively evaluating emulsion grain dispersity originally developed for nontabular grain emulsions and later applied to tabular grain emulsions provide a measure of the dispersity of ECDs. Given the essentially isometric shapes of most nontabular grains, dispersity measurements based on ECDs were determinative. As first the nonconforming grain populations and then the diameter dispersity of the tabular grains themselves have been restricted in tabular grain emulsions, those skilled in the art have begun to address now a third variance parameter of tabular grain emulsions which, unlike the first two, is not addressed by COV measurements. The importance of controlling variances in the thicknesses of tabular grains has been gradually realized. It is theoretically possible, for example, to have two tabular grain emulsions with the same measured COV that nevertheless differ significantly in grain to grain variances, since COVs are based exclusively on the ECDs of the tabular grains and do not take variances in grain thicknesses into account.
Although not developed to the level of a quantitative statistical measurement technique, those precipitating tabular grain emulsions have observed that the thickness dispersity of tabular grain emulsions can be visually observed and qualitatively compared as a function of their differing grain reflectances. When white light is directed toward a tabular grain population observed through a microscope, the light reflected from each tabular grain is reflected from its upper and lower major crystal faces. By traveling a slightly greater distance (twice the thickness of a tabular grain) light reflected from a bottom major crystal surface is phase shifted with respect to that reflected from a top major crystal surface. Phase shifting reduces the observed reflection of differing wavelengths to differing degrees, resulting in tabular grains of differing wavelengths exhibiting differing hues. An illustration of this effect is provided in Research Disclosure, Vol. 253, May 1985, Item 25330. In the tabular grain thickness range of from about 0.08 to 0.30 .mu.m distinct differences in hue of reflected light are often visually detectable with thickness differences of 0.01 .mu.m or less. The same differences in hue can be observed when overlapping grains have a combined thickness in the indicated range. Tabular grain emulsions with low tabular grain thickness dispersities can be qualitatively distinguished by the proportions of tabular grains with visually similar hues. Rigorous quantitative determinations of tabular grain thickness dispersities determined from reflected hues have not yet been reported.
Although there has been general photographic interest in reducing the dispersity of the grains in tabular grain emulsions, in dye image reversal photographic elements Sowinski et al U.S. Pat. No. 4,656,122 has reported increased threshold imaging speeds, reduced toe region density, increased maximum density and increased contrast to result from blending a smaller grain emulsion with a tabular grain emulsion, thereby increasing the overall dispersity of the resulting emulsion.